Pressure Point ST-09 is situated right over the carotid sinus. The carotid sinus is a Baroreceptor, whose purpose is to detect an increase in blood pressure. When it detects an increase, it sends a signal to the vasomotor center of the via the vagus nerve. This initiates a Vasodilatation, and slowing of the heart rate to lower the blood pressure to normal.
When struck this pressure points is struck there is an immediate, violent and dangerous response from the brain. The blood pressure will drop quickly causing a "vascular faint" or knockout. This can be accomplished with a low to medium strike.
A harder strike can cause serious damage to the nervous and arterial systems of the area which will result in death. ST-09 is also Dim Mak points because an energy strike to this part of the system, both nervous and vascular, will cause enough "dysfunction" to result in death overtime.
Carotid Sinus Reflex:
The serious martial artist should be concerned about Carotid Sinus Reflex when it comes to this pressure point and ST-10. The effect is temporary loss of consciousness or a knockout that sometimes accompanies convulsive seizures. This is because of the power of the carotid sinus reflex when pressure builds up in one or both carotid sinuses.